The present invention relates to a position transducer. With robots or numerical control machines, it is essential to know, at all times, the absolute position of each mobile element according to at least one respective degree of freedom (axis).
This is normally achieved using incremental position transducers (encoders), which are position, conveniently angular position, sensors that supply two signals for measuring the direction and rotation angle of a rotary shaft. The information supplied by the encoders is coded in the form of a sequence of pulses equal to the number of elementary increments of the rotary shaft.
The pulse sequences are transmitted via two signals with a 90.degree. phase lead or delay depending on the rotation direction of the encoder shaft. The actual position of the rotary shaft is determined by a measuring device which decodes the pulses supplied by the encoder. The measurement relates to the starting position of the shaft, and therefore requires a further reference for obtaining absolute information. A third pulse is therefore emitted by the encoder when a reference position, known as the zero position of the shaft, is reached. In this way, if a counter for measuring the signals from the encoder is reset by the zero pulse, the information supplied by the encoder is rendered absolute within one turn of the shaft.
In the case of rotary shafts involving a large number of turns (e.g. drive shafts with reduction gears, winders, etc..), provision must be made for a position sensor capable of determining which turn of the encoder relates to the zero reference angle and, therefore, which zero pulse the counter is to be reset by.
When the machine equipped with the encoder is started up, the transducer shaft must therefore be set to a predetermined zero position for resetting the counter, to do which, the mechanical assembly connected to the encoder shaft must be set to a position corresponding to the zero position of the encoder.
This is a time-consuming operation for which no information is available concerning the position of the moving parts of the machine.
What is more, in the case of complex systems, such as multiple-axis robots, operation is seriously impaired due to the downtime involved in setting up the machine, and the safety hazards posed by working on the machine with the position control deactivated.
Absolute, e.g. multirotation, encoders exist which supply absolute position information covering the entire travel of the shaft, which information is maintained even when supply to the encoders is cut off. Such devices, however, are of fairly complex mechanical design and expensive to produce.